1. Field of the Invention
The present invention relates to an insulating substrate, which is suitable for use as a circuit board, and a manufacturing method for the insulating substrate, and a multilayer wiring board and a manufacturing method for the multilayer wiring board.
2. Description of the Related Art
While multilayer wiring boards using ceramics as their insulating substrates have conventionally been in wide use, in keeping with the recent demand for multilayer wiring boards provided with various functions, multilayer wiring boards fabricated by the use of a combination of ceramics materials of different types have been proposed to date. For example, there are known a multilayer wiring board that offers high strength by using a combination of a low-strength ceramic insulating layer and a high-strength ceramic insulating layer, and a multilayer wiring board that allows incorporation of a high-capacitance capacitor by using a laminated body composed of low-dielectric constant insulating layers having sandwiched therebetween a high-dielectric constant insulating layer.
In such a multilayer wiring board, to prevent generation of cracks and delamination in ceramics materials, in general, the selection and control of the characteristics of insulating-layer materials are made carefully to make the ceramic insulating layers of different types uniform in terms of firing shrinkage and thermal expansion coefficient.
Moreover, in order to carry out a reduction of costs for manufacturing a multilayer wiring board and enhancement of the dimensional accuracy of an electrode formed on a multilayer wiring board (insulating substrate) or highly accurate mounting of constituent components, it has recently been demanded that the firing shrinkage of an insulating substrate should be reduced in the plane direction (X-Y direction) thereof, and warpage and deformation resulting from shrinking-behavior variations occurring in the course of firing should be suppressed. Unfortunately, multilayer wiring boards of a related art, however, failed to meet such a demand.
In order to satisfy such a requirement, the following methods have been developed in recent years. Namely, on the one hand, in accordance with a so-called pressure-firing technique, an unfired laminated body composed of insulating green sheets (inclusive of a plurality of sheets to be formed into insulating layers of different types) is fired while being pressurized through an Al2O3 sintered plate so as to increase the degree of firing shrinking in the thickness direction (Z direction) thereof. Thereby, a wiring board having a desired insulating substrate is fabricated. On the other hand, on the surface of a green-sheet laminated body is disposed an unfired ceramic layer which will not be sintered at a temperature at which the laminated body is burned, so that the green-sheet laminated body is arrested by the unfired ceramic layer. Thereby, the shrinking of the laminated body is so controlled that it occurs only in the thickness direction thereof. Afterwards, the unfired ceramic layer is removed (for example, refer to U.S. Pat. No. 2,554,415). In either method, a conductor paste for forming a wiring pattern is applied to the surface of the green sheet, or is charged inside the green sheet. Then, by firing the laminated body, an insulating substrate and a wiring conductor layer can be obtained at the same time (co-firing).
However, these methods have encountered the following problems. Namely, the former adopting the pressure-firing technique necessitates not only a warpage-free Al2O3 sintered plate but also specially-devised pressurizing means, whereas the latter exploiting the arresting action of the unfired ceramic layer (restrictive firing) necessitates a step for removing the unfired ceramic layer after the firing process is over, which is liable to result in an undesirable increase in the number of manufacturing process steps.
In light of the foregoing, the following circuit board manufacturing method has been proposed to date (for example, refer to Japanese Unexamined Patent Publication JP-A 2002-261443). Namely, in firing a laminated body composed of two kinds of insulating green sheets having different firing shrinking starting temperatures at one time, their firing shrinking starting temperatures are so controlled that, at the point in time when one green sheet having a higher firing shrinking starting temperature begins to shrink, the other green sheet having a lower firing shrinking starting temperature has already arrived at 90% or more of a predetermined final firing volume shrinking amount. This makes it possible to suppress dimensional changes in a resulting circuit board.
In the circuit board manufacturing method disclosed in JP-A 2002-261443, in firing a laminated body composed of two kinds of ceramic moldings having different firing shrinking starting temperatures at one time, it is necessary to control their firing shrinking starting temperatures in such a manner that, at the point in time when one insulating layer having a higher firing shrinking starting temperature begins to shrink, the other insulating layer having a lower firing shrinking starting temperature has already arrived at 90% or more of a predetermined final firing volume shrinking amount. This makes it possible to suppress dimensional changes in a resulting circuit board. However, on the negative side, this method pays heed only to the control of firing shrinking starting temperatures. This could give rise to problems such as occurrence of appreciable shrinking-behavior variations under restriction and a failure of ensuring sufficiently low shrinkage.
According to the manufacturing method disclosed in JP-A 2002-261443, dimensional-change control can be exercised with ease. However, only with this advantage, it is difficult to ensure that the plane direction-wise shrinkage of the substrate becomes practically zero, or undesirable shrinkage variations are inevitable. Furthermore, in forming the green sheet having a higher firing shrinking starting temperature and the green sheet having a lower firing shrinking starting temperature, while the characteristics of the sheet materials can be changed according to purposes, it is necessary to make adjustment to the shrinking behaviors of the sheets individually. This imposes limitations on material design and thus the characteristics of materials cannot be changed greatly. Accordingly, in order to attain improved capability more effectively, another insulating green sheet which is free from shrinking-behavior limitations needs to be prepared for use. Moreover, in a wiring conductor layer obtained by restrictive firing, its plane direction-wise shrinkage is restrained as with the case of the substrate. It is thus inevitable that most of the total necessary firing shrinkage of the wiring conductor layer will be compensated for by the thickness direction-wise shrinkage thereof. As a result, asperities tend to appear at the interface between the wiring conductor layer and the insulating layer, in consequence whereof there results an increased conductor resistance and poor interface conductivity. This leads up to electrical characteristic degradation in a resulting insulator layer with a built-in capacitor or the like function.
Also proposed is the following method (for example, refer to JP-A 2002-290037). Namely, in firing a laminated body composed of two kinds of ceramic moldings having different firing shrinking starting temperatures and a wiring layer at one time, the volume shrinkage and shrinking starting temperature of a conductor layer is so controlled as to prevent cracks and delamination from occurring in the vicinity of the interface of the conductor layer.
However, in recent years, circuit boards have been becoming increasingly lower in profile and yet higher in performance. As a natural consequence of this trend, slimness has been sought after in a ceramic molding for use therein, and correspondingly the volumetric ratio of a conductor layer present in the ceramic molding to the ceramic molding has been increasing. In the end, the ceramic molding and the conductor layer have been prone to vary in firing shrinking behavior, which leads up to appreciable warpage. This problem was taken no notice of in JP-A 2002-261443 and JP-A 2002-290037.